Plasma doping with enhanced charge neutralization

ABSTRACT

A plasma doping apparatus includes a pulsed power supply that generates a pulsed waveform having a first period with a first power level and a second period with a second power level. A plasma source generates a pulsed plasma with the first power level during the first period and with the second power level during the second period. A bias voltage power supply generates a bias voltage waveform at an output that is electrically connected to a platen which supports a substrate. The bias voltage waveform having a first voltage during a first period and second voltage with a negative potential that attract ions in the plasma to the substrate for plasma doping during a second period. At least one of the first and second power levels of the RF waveform is chosen to at least partially neutralize charge accumulating on the substrate.

The section headings used herein are for organizational purposes onlyand should not to be construed as limiting the subject matter describedin the present application.

BACKGROUND OF THE INVENTION

Plasma processing has been widely used in the semiconductor and otherindustries for many decades. Plasma processing is used for tasks such ascleaning, etching, milling, and deposition. More recently, plasmaprocessing has been used for doping. Plasma doping is sometimes referredto as PLAD or plasma immersion ion implantation (PIII). Plasma dopingsystems have been developed to meet the doping requirements of somemodern electronic and optical devices.

Plasma doping is fundamentally different from conventional beam-line ionimplantation systems that accelerate ions with an electric field andthen filter the ions according to their mass-to-charge ratio to selectthe desired ions for implantation. In contrast, plasma doping systemsimmerse the target in a plasma containing dopant ions and bias thetarget with a series of negative voltage pulses. The electric fieldwithin the plasma sheath accelerates ions toward the target therebyimplanting the ions into the target surface.

Plasma doping systems for the semiconductor industry generally require avery high degree of process control. Conventional beam-line ionimplantation systems that are widely used in the semiconductor industryhave excellent process control and also excellent run-to-run uniformity.Conventional beam-line ion implantation systems provide highly uniformdoping across the entire surface of state-of-the art semiconductorsubstrates.

In general, the process control of plasma doping systems is not as goodas conventional beam-line ion implantation systems. In many plasmadoping systems, charge tends to accumulate on the substrate being plasmadoped. This charge build-up can result in the development of arelatively high potential voltage on the substrate that can cause dopingnon-uniformities, arcing, and device damage.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, in accordance with preferred and exemplary embodiments,together with further advantages thereof, is more particularly describedin the following detailed description, taken in conjunction with theaccompanying drawings. The drawings are not necessarily to scale,emphasis instead generally being placed upon illustrating principles ofthe invention.

FIG. 1 illustrates a plasma doping system with charge neutralizationaccording to the present invention.

FIG. 2A illustrates a prior art waveform generated by the RF sourcehaving a single amplitude that can cause charge accumulation on thesubstrate under some conditions.

FIG. 2B illustrates a waveform generated by the bias voltage supply thatapplies a negative voltage to the substrate during plasma doping toattract ions in the plasma.

FIG. 3A illustrates a waveform generated by the RF source according tothe present invention that has multiple amplitudes for at leastpartially neutralizing charge accumulation on the substrate.

FIG. 3B illustrates a waveform generated by the bias voltage supplyaccording to the present invention that applies a negative voltage tothe substrate during plasma doping to attract ions.

FIG. 3C illustrates a waveform generated by the bias voltage supplyaccording to the present invention that applies a negative voltage tothe substrate during plasma doping to attract ions and that applies apositive voltage to the substrate after plasma doping is terminated toassist in neutralizing charge on the substrate.

FIGS. 4A-C illustrates a waveform generated by the RF source andwaveforms generated by the bias voltage supply according to the presentinvention that are similar to the waveforms described in connection withFIGS. 3A-3C, but that are displaced in time so as to plasma dope withboth the first and the second power level P_(RF1), P_(RF2).

FIGS. 5A-C illustrate a waveform generated by the RF source with avariable frequency and waveforms generated by the bias voltage supplyaccording to another embodiment of the present invention.

DETAILED DESCRIPTION

Reference in the specification to “one embodiment” or “an embodiment”means that a particular feature, structure, or characteristic describedin connection with the embodiment is included in at least one embodimentof the invention. The appearances of the phrase “in one embodiment” invarious places in the specification are not necessarily all referring tothe same embodiment.

It should be understood that the individual steps of the methods of thepresent invention may be performed in any order and/or simultaneously aslong as the invention remains operable. Furthermore, it should beunderstood that the apparatus and methods of the present invention caninclude any number or all of the described embodiments as long as theinvention remains operable.

The present teachings will now be described in more detail withreference to exemplary embodiments thereof as shown in the accompanyingdrawings. While the present teachings are described in conjunction withvarious embodiments and examples, it is not intended that the presentteachings be limited to such embodiments. On the contrary, the presentteachings encompass various alternatives, modifications and equivalents,as will be appreciated by those of skill in the art. Those of ordinaryskill in the art having access to the teachings herein will recognizeadditional implementations, modifications, and embodiments, as well asother fields of use, which are within the scope of the presentdisclosure as described herein. For example, it should be understoodthat the methods for neutralizing charge in a plasma doping systemaccording to the present invention can be used with any type of plasmasource.

Many plasma doping systems operate in a pulsed mode of operation where aseries of pulses is applied to the plasma source to generate a pulsedplasma. Also, a series of pulses can be applied to the substrate beingplasma doped during the on-periods of the plasma source pulses to biasthe substrate to attract ions for implantation. In the pulsed mode ofoperation, charge tends to accumulate on the substrate being plasmadoped during the on-period of the plasma source pulses. When the dutycycle of the plasma source pulses is relatively low (i.e. less thanabout 25%), the charge tends to be efficiently neutralized by electronsin the plasma.

However, there is currently a need to perform plasma doping in a pulsedmode of operation with relatively high duty cycles (i.e. duty cyclesabove about a 25%). Such higher duty cycles are necessary to achieve thedesired throughputs and to maintain doping levels that are required forsome modern devices. For example, it is desirable to perform poly gatedoping and counter doping of some state-of-the art devices by plasmadoping with a duty cycle greater than 25%.

As the duty cycle is increased above about 25%, there is a shorterperiod of time where the charge on the substrate being plasma doped canbe neutralized during the pulse-off period of the plasma source.Consequently, charge accumulation or charge build up can occur on thesubstrate being plasma doped, which results in the development of arelatively high potential voltage on the substrate being plasma dopedthat can cause doping non-uniformities, arcing, and device damage. Forexample, thin gate dielectrics can be easily damaged by excess chargebuild up.

The present invention relates to methods and apparatus for neutralizingcharge during plasma doping. The method and apparatus of the presentinvention allow implants to be performed at higher duty cycles byreducing the probability of damage caused by charging effects. Inparticular, a plasma doping apparatus according to the present inventionincludes a RF power supply that varies the RF power applied to theplasma source to at least partially neutralize charge accumulationduring plasma doping. In addition, the bias voltage to the substratebeing plasma doped can be varied to at least partially neutralize chargeaccumulation. Furthermore, the relative timing of the RF power pulsesapplied to the plasma source and the bias voltage applied to thesubstrate being plasma doped can be varied to at least partiallyneutralize charge accumulation.

More specifically, a plasma implantation system according to the presentinvention includes a RF power supply that varies the RF power applied tothe plasma source to at least partially neutralize charge accumulationduring plasma doping. In various embodiments single or multiple RF powersupplies are used to independently power the plasma source and the biasthe substrate being plasma doped so as to at least partially neutralizecharge during plasma doping. Also, in various embodiments, the RF powerapplied to the plasma source and the bias voltage applied to thesubstrate during plasma doping are applied at relative times to at leastpartially neutralize charge during plasma doping.

In addition to neutralizing charge, the method and apparatus of thepresent invention can precisely control at least one of the power to theRF source and the bias applied to the substrate during periods where theplasma doping is terminated (i.e. pulse-off period) in order to improvethe retained dose. The resulting improvement in retained dose will helpto reduce implant time and thus will increase throughput. Also, inaddition to neutralizing charge, the method and apparatus of the presentinvention can precisely control at least one of the power to the RFsource and the bias applied to the substrate during periods where theplasma doping is terminated in order to achieve knock-on type implantmechanisms that obtain better sidewall coverage.

FIG. 1 illustrates a plasma doping system 100 with charge neutralizationaccording to the present invention. It should be understood that this isonly one of many possible designs plasma doping systems that can performion implantation with charge neutralization according to the presentinvention. The plasma doping system 100 includes an inductively coupledplasma source 101 having both a planar and a helical RF coil and aconductive top section. A similar RF inductively coupled plasma sourceis described in U.S. patent application Ser. No. 10/905,172, filed onDec. 20, 2004, entitled “RF Plasma Source with Conductive Top Section,”which is assigned to the present assignee. The entire specification ofU.S. patent application Ser. No. 10/905,172 is incorporated herein byreference. The plasma source 101 shown in the plasma doping system 100is well suited for plasma doping applications because it can provide ahighly uniform ion flux and the source also efficiently dissipates heatgenerated by secondary electron emissions.

More specifically, the plasma doping system 100 includes a plasmachamber 102 that contains a process gas supplied by an external gassource 104. The external gas source 104, which is coupled to the plasmachamber 102 through a proportional valve 106, supplies the process gasto the chamber 102. In some embodiments, a gas baffle is used todisperse the gas into the plasma source 101. A pressure gauge 108measures the pressure inside the chamber 102. An exhaust port 110 in thechamber 102 is coupled to a vacuum pump 112 that evacuates the chamber102. An exhaust valve 114 controls the exhaust conductance through theexhaust port 110.

A gas pressure controller 116 is electrically connected to theproportional valve 106, the pressure gauge 108, and the exhaust valve114. The gas pressure controller 116 maintains the desired pressure inthe plasma chamber 102 by controlling the exhaust conductance and theprocess gas flow rate in a feedback loop that is responsive to thepressure gauge 108. The exhaust conductance is controlled with theexhaust valve 114. The process gas flow rate is controlled with theproportional valve 106.

In some embodiments, a ratio control of trace gas species is provided tothe process gas by a mass flow meter that is coupled in-line with theprocess gas that provides the primary dopant species. Also, in someembodiments, a separate gas injection means is used for in-situconditioning species. Furthermore, in some embodiments, a multi-port gasinjection means is used to provide gases that cause neutral chemistryeffects that result in across substrate variations.

The chamber 102 has a chamber top 118 including a first section 120formed of a dielectric material that extends in a generally horizontaldirection. A second section 122 of the chamber top 118 is formed of adielectric material that extends a height from the first section 120 ina generally vertical direction. The first and second sections 120, 122are sometimes referred to herein generally as the dielectric window. Itshould be understood that there are numerous variations of the chambertop 118. For example, the first section 120 can be formed of adielectric material that extends in a generally curved direction so thatthe first and second sections 120, 122 are not orthogonal as describedin U.S. patent application Ser. No. 10/905,172, which is incorporatedherein by reference. In other embodiment, the chamber top 118 includesonly a planer surface.

The shape and dimensions of the first and the second sections 120, 122can be selected to achieve a certain performance. For example, oneskilled in the art will understand that the dimensions of the first andthe second sections 120, 122 of the chamber top 118 can be chosen toimprove the uniformity of plasmas. In one embodiment, a ratio of theheight of the second section 122 in the vertical direction to the lengthacross the second section 122 in the horizontal direction is adjusted toachieve a more uniform plasma. For example, in one particularembodiment, the ratio of the height of the second section 122 in thevertical direction to the length across the second section 122 in thehorizontal direction is in the range of 1.5 to 5.5.

The dielectric materials in the first and second sections 120, 122provide a medium for transferring the RF power from the RF antenna to aplasma inside the chamber 102. In one embodiment, the dielectricmaterial used to form the first and second sections 120, 122 is a highpurity ceramic material that is chemically resistant to the processgases and that has good thermal properties. For example, in someembodiments, the dielectric material is 99.6% Al₂O₃ or AlN. In otherembodiments, the dielectric material is Yittria and YAG.

A lid 124 of the chamber top 118 is formed of a conductive material thatextends a length across the second section 122 in the horizontaldirection. In many embodiments, the conductivity of the material used toform the lid 124 is high enough to dissipate the heat load and tominimize charging effects that results from secondary electron emission.Typically, the conductive material used to form the lid 124 ischemically resistant to the process gases. In some embodiments, theconductive material is aluminum or silicon.

The lid 124 can be coupled to the second section 122 with a halogenresistant O-ring made of fluoro-carbon polymer, such as an O-ring formedof Chemrz and/or Kalrex materials. The lid 124 is typically mounted tothe second section 122 in a manner that minimizes compression on thesecond section 122, but that provides enough compression to seal the lid124 to the second section. In some operating modes, the lid 124 is RFand DC grounded as shown in FIG. 1.

In some embodiments, the chamber 102 includes a liner 125 that ispositioned to prevent or greatly reduce metal contamination by providingline-of-site shielding of the inside of the plasma chamber 102 frommetal sputtered by ions in the plasma striking the inside metal walls ofthe plasma chamber 102. Such liners are described in U.S. patentapplication Ser. No. 11,623,739, filed Jan. 16, 2007, entitled “PlasmaSource with Liner for Reducing Metal Contamination,” which is assignedto the present assignee. The entire specification of U.S. patentapplication Ser. No. 11/623,739 is incorporated herein by reference.

In various embodiments, the liner can be a one-piece or unitary plasmachamber liner, or a segmented plasma chamber liner. In many embodiments,the plasma chamber liner 125 is formed of a metal base material, such asaluminum. In these embodiments, at least the inner surface 125′ of theplasma chamber liner 125 includes a hard coating material that preventssputtering of the plasma chamber liner base material.

Some plasma doping processes generate a considerable amount ofnon-uniformly distributed heat on the inner surfaces of the plasmasource 101 because of secondary electron emissions. In some embodiments,the plasma chamber liner 125 is a temperature controlled plasma chamberliner 125. In addition, in some embodiments, the lid 124 comprises acooling system that regulates the temperature of the lid 124 andsurrounding area in order to dissipate the heat load generated duringprocessing. The cooling system can be a fluid cooling system thatincludes cooling passages in the lid 124 that circulate a liquid coolantfrom a coolant source.

A RF antenna is positioned proximate to at least one of the firstsection 120 and the second section 122 of the chamber top 118. Theplasma source 101 in FIG. 1 illustrates two separate RF antennas thatare electrically isolated from one another. However, in otherembodiments, the two separate RF antennas are electrically connected. Inthe embodiment shown in FIG. 1, a planar coil RF antenna 126 (sometimescalled a planar antenna or a horizontal antenna) having a plurality ofturns is positioned adjacent to the first section 120 of the chamber top118. In addition, a helical coil RF antenna 128 (sometimes called ahelical antenna or a vertical antenna) having a plurality of turnssurrounds the second section 122 of the chamber top 118.

In some embodiments, at least one of the planar coil RF antenna 126 andthe helical coil RF antenna 128 is terminated with a capacitor 129 thatreduces the effective antenna coil voltage. The term “effective antennacoil voltage” is defined herein to mean the voltage drop across the RFantennas 126, 128. In other words, the effective coil voltage is thevoltage “seen by the ions” or equivalently the voltage experienced bythe ions in the plasma.

Also, in some embodiments, at least one of the planar coil RF antenna126 and the helical coil RF antenna 128 includes a dielectric layer 134that has a relatively low dielectric constant compared to the dielectricconstant of the Al₂O₃ dielectric window material. The relatively lowdielectric constant dielectric layer 134 effectively forms a capacitivevoltage divider that also reduces the effective antenna coil voltage. Inaddition, in some embodiments, at least one of the planar coil RFantenna 126 and the helical coil RF antenna 128 includes a Faradayshield 136 that also reduces the effective antenna coil voltage.

A RF source 130, such as a RF power supply, is electrically connected toat least one of the planar coil RF antenna 126 and helical coil RFantenna 128. In many embodiments, the RF source 130 is coupled to the RFantennas 126, 128 by an impedance matching network 132 that matches theoutput impedance of the RF source 130 to the impedance of the RFantennas 126, 128 in order to maximize the power transferred from the RFsource 130 to the RF antennas 126, 128. Dashed lines from the output ofthe impedance matching network 132 to the planar coil RF antenna 126 andthe helical coil RF antenna 128 are shown to indicate that electricalconnections can be made from the output of the impedance matchingnetwork 132 to either or both of the planar coil RF antenna 126 and thehelical coil RF antenna 128.

In some embodiments, at least one of the planar coil RF antenna 126 andthe helical coil RF antenna 128 is formed such that it can be liquidcooled. Cooling at least one of the planar coil RF antenna 126 and thehelical coil RF antenna 128 will reduce temperature gradients caused bythe RF power propagating in the RF antennas 126, 128.

In some embodiments, the plasma source 101 includes a plasma igniter138. Numerous types of plasma igniters can be used with the plasmasource 101. In one embodiment, the plasma igniter 138 includes areservoir 140 of strike gas, which is a highly-ionizable gas, such asargon (Ar), which assists in igniting the plasma. The reservoir 140 iscoupled to the plasma chamber 102 with a high conductance gasconnection. A burst valve 142 isolates the reservoir 140 from theprocess chamber 102. In another embodiment, a strike gas source isplumbed directly to the burst valve 142 using a low conductance gasconnection. In some embodiments, a portion of the reservoir 140 isseparated by a limited conductance orifice or metering valve thatprovides a steady flow rate of strike gas after the initialhigh-flow-rate burst.

A platen 144 is positioned in the process chamber 102 a height below thetop section 118 of the plasma source 101. The platen 144 holds asubstrate 146 for plasma doping. In many embodiments, the substrate 146is electrically connected to the platen 144. In the embodiment shown inFIG. 1, the platen 144 is parallel to the plasma source 101. However, inone embodiment of the present invention, the platen 144 is tilted withrespect to the plasma source 101.

A platen 144 is used to support a substrate 146 or other workpieces forprocessing. In some embodiments, the platen 144 is mechanically coupledto a movable stage that translates, scans, or oscillates the substrate146 in at least one direction. In one embodiment, the movable stage is adither generator or an oscillator that dithers or oscillates thesubstrate 146. The translation, dithering, and/or oscillation motionscan reduce or eliminate shadowing effects and can improve the uniformityof the ion beam flux impacting the surface of the substrate 146.

A bias voltage power supply 148 is electrically connected to the platen144. The bias voltage power supply 148 is used to bias the platen 144and the substrate 146 so that dopant ions in the plasma are extractedfrom the plasma and impact the substrate 146. The bias voltage powersupply 148 can be a DC power supply, a pulsed power supply, or a RFpower supply. In plasma doping apparatus according the presentinvention, the bias voltage power supply 148 has an output that isindependent of the output of the RF source 130 that powers at least oneof the planar coil RF antenna 126 and helical coil RF antenna 128.However, the bias voltage power supply 148 and the RF source 130 canphysically be the same power supply as long as the bias voltage outputis independent of the RF source output.

A controller 152 is used to control the RF power supply 130 and the biasvoltage power supply 148 to generate a plasma and to bias the substrate146 so as to at least partially neutralize charge accumulation duringplasma doping according to the present invention. The controller 152 canbe part of the power supplies 130, 148 or can be a separate controllerthat is electrically connected to control inputs of the power supplies130, 148. The controller 152 controls the RF power supply 130 so thatpulses are applied to either or both of the planar coil RF antenna 126and the helical coil RF antenna 128 with at least two differentamplitudes. Also, the controller 152 controls the RF power supply 130and the bias voltage power supply 148 so that the pulses are applied toeither or both of the planar coil RF antenna 126 and the helical coil RFantenna 128 and to the substrate at relative times that at leastpartially neutralize charge accumulation during plasma doping accordingto the present invention.

One skilled in the art will appreciate that the there are many differentpossible variations of the plasma source 101 that can be used with thefeatures of the present invention. See for example, the descriptions ofthe plasma sources in U.S. patent application Ser. No. 10/908,009, filedApr. 25, 2005, entitled “Tilted Plasma Doping.” Also see thedescriptions of the plasma sources in U.S. patent application Ser. No.11/163,303, filed Oct. 13, 2005, entitled “Conformal Doping Apparatusand Method.” Also see the descriptions of the plasma sources in U.S.patent application Ser. No. 11/163,307, filed Oct. 13, 2005, entitled“Conformal Doping Apparatus and Method.” In addition, see thedescriptions of the plasma sources in U.S. patent application Ser. No.11/566,418, filed Dec. 4, 2006, entitled “Plasma Doping withElectronically Controllable implant Angle.”The entire specification ofU.S. patent application Ser. Nos. 10/908,009, 11/163,303, 11/163,307 and11/566,418 are herein incorporated by reference.

In operation, the controller 152 instructs the RF source 130 to generateRF currents that propagate in at least one of the RF antennas 126 and128. That is, at least one of the planar coil RF antenna 126 and thehelical coil RF antenna 128 is an active antenna. The term “activeantenna” is herein defined as an antenna that is driven directly by apower supply. In many embodiments of the plasma doping apparatus of thepresent invention, the RF source 130 operates in a pulsed mode. However,the RF source 130 can also operate in the continuous mode.

In some embodiments, one of the planar coil antenna 126 and the helicalcoil antenna 128 is a parasitic antenna. The term “parasitic antenna” isdefined herein to mean an antenna that is in electromagneticcommunication with an active antenna, but that is not directly connectedto a power supply. In other words, a parasitic antenna is not directlyexcited by a power supply, but rather is excited by an active antenna,which in the present invention is one of the planar coil antenna 126 andthe helical coil antenna 128 powered by the RF source 130. In someembodiments of the invention, one end of the parasitic antenna iselectrically connected to ground potential in order to provide antennatuning capabilities. In this embodiment, the parasitic antenna includesa coil adjuster 150 that is used to change the effective number of turnsin the parasitic antenna coil. Numerous different types of coiladjusters, such as a metal short, can be used.

The RF currents in the RF antennas 126, 128 then induce RF currents intothe chamber 102. The RF currents in the chamber 102 excite and ionizethe process gas so as to generate a plasma in the chamber 102. Theplasma chamber liner 125 shields metal sputtered by ions in the plasmafrom reaching the substrate 146.

The controller 152 also instructs the bias voltage power supply 148 tobias the substrate 146 with a negative voltage that attract ions in theplasma towards the substrate 146. During the negative voltage pulses,the electric field within the plasma sheath accelerates ions toward thesubstrate 146 which implants the ions into the surface of the substrate146. In some embodiments, a grid is used to extract ions in the plasmatowards the substrate 146.

When the RF source 130 and the bias voltage power supply 148 areoperated in the pulse mode under some processing conditions, such aswith relatively high duty cycles, charge can accumulate on the substrate146. Charge accumulation can result in the development of a relativelyhigh potential voltage on the substrate 146 being plasma doped that cancause doping non-uniformities, arcing, and device damage.

FIG. 2A illustrates a prior art waveform 200 generated by the RF source130 having a single amplitude that can cause charge accumulation on thesubstrate 146 under some conditions. The waveform 200 is at groundpotential until the plasma is generated with a pulse having a powerlevel P_(RF) 202. The power level P_(RF) 202 is chosen to be suitablefor plasma doping. The pulse terminates after the pulse period T_(P) 204and then returns to ground potential. The waveform then periodicallyrepeats.

FIG. 2B illustrates a waveform 250 generated by the bias voltage supply148 according to the present invention that applies a negative voltage252 to the substrate 146 during plasma doping to attract ions in theplasma. The negative voltage 252 is applied during the period T₁ 254when the waveform 200 generated by the RF source 130 has a power equalto the power level P_(RF) 202. The waveform 200 is at ground potentialduring the period T₂ 256 when the plasma doping is terminated. Atrelatively high duty cycles (i.e. greater than about 25%), charge tendsto accumulate on the substrate 146 during the pulse period T₁ 254 whenthe waveform 250 generated by the RF source 130 has a power equal to thepower level P_(RF) 202.

The methods and apparatus of the present invention allow plasma dopingimplants to be performed at higher duty cycles by reducing theprobability of damage caused by charging effects. There are numerousmethods according to the present invention to power the plasma source101 and to bias the substrate 146 being process to at least partiallyneutralize charge accumulation on the substrate 146.

FIG. 3A illustrates a waveform 300 generated by the RF source 130according to the present invention that has multiple amplitudes for atleast partially neutralizing charge accumulation on the substrate 146.The waveform 300 is pulsed and has a first 302 and second power level304 indicated in the figure as P_(RF1) and P_(RF2), respectively.However, it should be understood that waveforms with more than twoamplitudes can be used in the methods of the present invention to atleast partially neutralize charge accumulation on the substrate 146. Itshould also be understood that the waveforms may or may not havediscrete amplitudes. For example, the waveforms can be continuouslychanging. That is, in some embodiments, the waveforms can ramp (i.e.have positive and negative slopes) linearly or nonlinearly.

The first power level P_(RF1) 302 is chosen to provide enough RF powerto at least partially neutralize charge accumulation on the substrate146 when the substrate 146 is not biased for plasma doping. The secondpower level P_(RF2) 304 is chosen to be suitable for plasma doping. Invarious embodiments, the waveform 300 generated by the RF source 130including the first and second power levels P_(RF1) 302, P_(RF2) 304 isapplied to one or both of the planar coil RF antenna 126 and the helicalcoil RF antenna 128 (see FIG. 1). In one specific embodiment, thewaveform 300 generated by the RF source 130 is applied to one of theplanar coil RF antenna 126 and the helical coil RF antenna 128 when itis at the first power levels P_(RF1) and is applied to the other of theplanar coil RF antenna 126 and the helical coil RF antenna 128 when itis at the second power levels P_(RF2). In another specific embodiment,the waveform 300 generated by the RF source 130 is applied to one of theplanar coil RF antenna 126 and the helical coil RF antenna 128 when ithas a first frequency and is applied to the other of the planar coil RFantenna 126 and the helical coil RF antenna 128 when it has a secondfrequency that is different from the first frequency as described inconnection with FIGS. 5A-5C.

The waveform 300 shown in FIG. 3A indicates that the first power levelP_(RF1) 302 is greater than the second power level P_(RF2) 304. However,in other embodiments, the first power level P_(RF1) 302 is less than thesecond power level P_(RF2) 304. Also, in some embodiments, the waveform300 includes a third power level that is zero or some relatively lowpower level when the substrate 146 is not biased for plasma doping.

The waveform 300 also indicates a first pulse period T_(P1) 306corresponding to the time period were the waveform 300 has a power equalto the first power level P_(RF1) 302 and a second pulse period T_(P2)308 corresponding to the time period were the waveform has a power equalto the second power level P_(RF2) 304. The total multi-amplitude pulseperiod for the waveform 300 T_(Total) 310 is the combination of thefirst pulse period T_(P1) 306 and the second pulse period T_(P2) 308.For example, in one embodiment, the first and second pulse periodsT_(P1) 306, T_(P2) 308 are both in the range of 30-500 μs and the totalpulse period T_(Total) 310 is in the range of 60 μs-1 ms. In otherembodiments, the total pulse period T_(Total) 310 can be on order of 1ms or greater.

FIG. 3A indicates that the frequency of the waveform 300 during thefirst pulse period T_(P1) 306 is the same as the frequency of thewaveform 300 during the second pulse period T_(P2) 308. However, itshould be understood that in various embodiments, the frequency of thewaveform 300 during the first pulse period T_(P1) 306 can be differentfrom the frequency of the waveform 300 during the second pulse periodT_(P2) 308 as described in connection with FIGS. 5A-5C. In addition, thefrequency of the waveform can be changed within at least one of thefirst and the second pulse periods T_(P1), 306, T_(P2), 308.

Thus, in some embodiments, the waveform 300 includes both multiplefrequencies and multiple amplitudes that are chosen to at leastpartially neutralize charge accumulation during plasma doping. Inaddition, in some embodiments, the waveform 300 includes both multiplefrequencies and multiple amplitudes that are chosen to improve theretained dose as described herein. Furthermore, in some embodiments, thewaveform 300 includes both multiple frequencies and multiple amplitudesthat are chosen to assist in creating knock-on implants as describedherein.

FIG. 3B illustrates a waveform 350 generated by the bias voltage supply148 according to the present invention that applies a negative voltage352 to the substrate 146 during plasma doping to attract ions. Thenegative voltage 352 is applied during the second pulse period T_(P2)308 when the waveform 350 generated by the RF source 130 has a powerequal to the second power level P_(RF2) 304. The waveform 350 is atground potential during the first pulse period T_(P1) 306 when theplasma doping is terminated and the waveform 300 has a power equal tothe first power level P_(RF1) 302.

Applying a waveform to the plasma source 101 with two different powerlevels where the first power level P_(RF1) 302 is applied by the RFsource 130 during the period 306 T_(P1) 306 when the waveform 350generated by the bias voltage supply 148 is at ground potential willassist in neutralizing charge accumulated on the substrate 146.Electrons in the corresponding plasma will neutralize at least some ofthe charge accumulated on the substrate 146.

FIG. 3C illustrates a waveform 360 generated by the bias voltage supply148 according to the present invention that applies a negative voltage362 to the substrate 146 during plasma doping to attract ions and thatapplies a positive voltage 364 to the substrate 146 after plasma dopingis terminated to assist in neutralizing charge on the substrate 146. Thenegative voltage 362 is applied during the second pulse period T_(P2)308 when the waveform 300 generated by the RF source 130 has a powerequal to the second power level P_(RF2) 304. The waveform 360 is at apositive potential during the first pulse period T_(P1) 306 when thewaveform 300 generated by the RF source 130 has a power equal to thefirst power level P_(RF1) 302.

Applying a waveform to the plasma source 101 with two different powerlevels where the first power level P_(RF1) 302 is applied by the RFsource 130 during the first period 306 T_(P1) 306 when the waveform 360generated by the bias voltage supply 148 is at a positive potential willassist in neutralizing charge accumulated on the substrate 146.Electrons in the corresponding plasma will neutralize at least some ofthe charge accumulated on the substrate 146. In addition, the positivevoltage 364 applied the substrate 146 will also neutralize at least someof the charge accumulated on the substrate 146.

FIGS. 4A-C illustrate a waveform 400 generated by the RF source 130 andwaveforms 402, 404 generated by the bias voltage supply 148 according tothe present invention that are similar to the waveforms 300, 350, and360 described in connection with FIGS. 3A-3C, but that are displaced intime relative to the waveforms 300, 350, and 360 so as to plasma dopewith both the first and the second power level P_(RF1) 302, P_(RF2) 304.Changing the power generated by the RF source 130 during plasma dopingallows the user to more precisely control the amount of charge that isaccumulating on the surface of the substrate 146 during plasma doping.For example, increasing the power near the end of the second pulseperiod T_(P2) 308 will assist in neutralizing at least some of thecharge accumulated on the substrate 146.

FIGS. 5A-C illustrate a waveform 500 generated by the RF source 130 witha variable frequency and waveforms 502, 504 generated by the biasvoltage supply 148 according to another embodiment of the presentinvention. The waveform 500 is similar to the waveforms 300, 400described in connection with FIGS. 3 and 4. However, the RF powers inthe first and second pulse periods T_(P1) 306, T_(P2) 308 are the same,but the frequencies are different. Changing the frequency of thewaveform 500 changes the ion/electron density and, therefore, changesthe charge neutralization efficiency.

Thus, in one embodiment, the frequency of the waveform 500 in the firstpulse period T_(P1) 306 is different from the frequency of the waveform500 in the second pulse period T_(P2) 308 and these frequencies arechosen to at least partially neutralize charge accumulation duringplasma doping. The waveforms 502, 504 are similar to the waveforms 350and 360 that were described in connection with FIG. 3. However, in otherembodiments, the waveforms 502, 504 are displaced in time relative tothe waveform 500, similar to the waveforms 402, 404 that were describedin connection with FIG. 4.

In addition, in one aspect of the present invention, at least one of themultiple power levels generated by the RF source 130, the frequency ofthe waveform 500 in at least one of the first and second pulse periodsT_(P1) 306, T_(P2) 308, and the relative timing of the waveform 500 withrespect to the waveforms generated by the bias voltage supply 148 arechosen to improve the retained dose on the substrate 146. For example,generating multiple power levels with the RF source 130 where one poweris generated by the RF source 130 when the bias voltage is at groundpotential allows the user to use less power during plasma doping becausesome plasma doping will occur between negative bias voltage steps. Usingless power during plasma doping will result in less deposition and,therefore, a higher retained dose. The operating pressure, gas flowrates, type of dilution gas, and plasma source power can also beselected to improve the retained dose.

In addition, in one aspect of the present invention, at least one of themultiple power levels generated by the RF source 130, the frequency ofthe waveform 500 in at least one of the first and second pulse periodsT_(P1) 306, T_(P2) 308, and the relative timing of the waveform 500 withrespect to the waveforms generated by the bias voltage supply 148 arechosen to obtain better sidewall coverage. For example, waveforms can begenerated by the RF source 130 with multiple power levels, multiplefrequencies, and with certain relative timings with respect to thewaveforms generated by the bias voltage supply 148 so as to createknock-on implants. The term “knock-on implant” is defined herein as arecoil implantation where a non-dopant species is implanted through thesurface layers of the substrate 146 to drive the dopant material intothe substrate 146.

The non-dopant species used for the knock-on implant can be a benignspecies. For example, inert ions, such as He, Ne, Ar, Kr and Xe, can beformed from an inert feed gas. In some embodiments, the mass of theinert ions is chosen to be similar to a mass of the desired dopant ions.The RF source 130 generates a RF power level that directs the inert ionstowards the substrate 146 with a sufficient energy to physically knockthe deposited dopant material into both the planar and nonplanarfeatures of the substrate 146 upon impact. Also, the operating pressure,gas flow rate, plasma source power, gas dilution, and duty cycle ofpulsed bias supply can be chosen to enhance knock-on implants.

One skilled in the art will appreciate that waveforms generated by theRF source 130 according to the present invention can have both multipleamplitudes and multiple frequencies and can have various relativetimings with respect to the waveforms generated by the bias voltagesupply 148. In fact, there are an almost infinite number of possiblewaveforms with multiple power levels and multiple frequencies that canbe generated by the RF source 130 and relative timing with respect tothe waveforms generated by the bias voltage supply 148 that will atleast partially neutralize charge according to the present invention. Inaddition, the retained dose can be improved by generating waveforms withthe RF source 130 with multiple power levels, multiple frequencies, andrelative timings with respect to the waveforms generated by the biasvoltage supply 148. Furthermore, knock-on implants can be enhanced bygenerating waveforms with the RF source 130 with multiple power levels,multiple frequencies, and relative timings with respect to the waveformsgenerated by the bias voltage supply 148. These waveforms can also havemany different duty cycles.

It should be understood that the methods for charge neutralizationaccording to the present invention can be used with numerous other typesof plasma doping apparatus. For example, the methods for chargeneutralization can be used with plasma doping apparatus that haveinductively coupled plasma (ICP) sources, helicon resonator plasmasources, microwave plasma sources, ECR plasma source, and capacitivecoupled plasma sources. In fact, any type of plasma source that can beoperated in a pulsed mode can be used to perform the methods of thepresent invention.

Equivalents

While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives, modifications and equivalents, as willbe appreciated by those of skill in the art, may be made therein withoutdeparting from the spirit and scope of the invention.

1. A plasma doping apparatus comprising: a. a pulsed power supply thatgenerates a pulsed waveform at an output, the pulsed waveform having atleast a first period with a first power level and a second period with asecond power level; b. a plasma source having an electrical input thatis electrically connected to the output of the pulsed power supply, theplasma source generating a pulsed plasma with the first power levelduring the first period and with the second power level during thesecond period; c. a platen that supports a substrate for plasma doping;and d. a bias voltage power supply having an output that is electricallyconnected to the platen, the bias voltage power supply generating a biasvoltage waveform having a first voltage during a first period and asecond voltage with a negative potential that attract ions in the plasmato the substrate for plasma doping during a second period, at least oneof the first and second power levels of the RF waveform being chosen toat least partially neutralize charge accumulating on the substrate. 2.The plasma doping apparatus of claim 1 wherein the first voltage in thebias voltage waveform is at ground potential.
 3. The plasma dopingapparatus of claim 1 wherein the first voltage in the bias voltagewaveform has a positive potential that at least partially neutralizecharge accumulating on the substrate.
 4. The plasma doping apparatus ofclaim 1 wherein the first and second periods of the pulsed waveformgenerated by the pulsed power supply are substantially the same as thefirst and second periods of the bias voltage waveform generated by thebias voltage power supply.
 5. The plasma doping apparatus of claim 1wherein the pulsed waveform generated by the pulsed power supply issynchronized to the bias voltage waveform generated by the bias voltagepower supply.
 6. The plasma doping apparatus of claim 1 wherein thepulsed waveform generated by the pulsed power supply includes a thirdperiod having a third power level.
 7. The plasma doping apparatus ofclaim 6 wherein the third power level is a zero power level.
 8. Theplasma doping apparatus of claim 1 wherein a relative timing of thepulsed waveform generated by the pulsed power supply and the biasvoltage waveform generated by the bias voltage power supply is chosen toat least partially neutralize charge accumulating on the substrate. 9.The plasma doping apparatus of claim 1 wherein a relative timing of thepulsed waveform generated by the pulsed power supply and the biasvoltage waveform generated by the bias voltage power supply is chosen sothat the pulsed waveform generated by the pulsed power supply changesfrom the first power level to the second power level during the secondperiod of the bias voltage waveform where the bias voltage waveform hasthe negative potential that attracts ions in the plasma to the substratefor plasma doping.
 10. The plasma doping apparatus of claim 1 whereinthe first and second power levels are chosen to increase retained dosein the substrate.
 11. The plasma doping apparatus of claim 1 wherein arelative timing of the pulsed waveform generated by the pulsed powersupply and the bias voltage waveform generated by the bias voltage powersupply is chosen to increase retained dose in the substrate.
 12. Theplasma doping apparatus of claim 1 wherein the first and second powerlevels are chosen to enhance knock-on type implant mechanisms so as toproduce a more conformal doping profile.
 13. The plasma doping apparatusof claim 1 wherein a relative timing of the pulsed waveform generated bythe pulsed power supply and the bias voltage waveform generated by thebias voltage power supply is chosen to enhance knock-on type implantmechanisms so as to produce a more conformal doping profile.
 14. Theplasma doping apparatus of claim 1 wherein a frequency of the pulsedwaveform in the first period is different from a frequency of the pulsedwaveform in the second period.
 15. A plasma doping apparatus comprising:a. a chamber that contains a process gas, the chamber comprising adielectric window that passes electromagnetic radiation; b. a RF sourcethat generates a RF waveform at an output, the RF waveform having atleast a first and a second power level; c. at least one RF antennahaving an input that is electrically connected to the output of the RFpower supply, the at least one RF antenna being positioned proximate tothe dielectric window so that the RF waveform electromagneticallycouples into the chamber to excite and ionize the process gas, therebyforming a plasma in the chamber; d. a platen that supports a substratefor plasma doping; and e. a bias voltage power supply having an outputthat is electrically connected to the platen, the bias voltage powersupply generating a bias voltage waveform at the output that includesnegative potential pulses that attract ions in the plasma to thesubstrate for plasma doping, at least one of the first and second powerlevels of the RF waveform and a relative timing of the RF waveform andthe bias voltage waveform being chosen to at least partially neutralizecharge accumulating on the substrate.
 16. The plasma doping apparatus ofclaim 15 wherein the at least one RF antenna comprises a vertical and ahorizontal antenna.
 17. The plasma doping apparatus of claim 15 whereinthe at least one RF antenna comprises an active antenna and a passiveantenna.
 18. The plasma doping apparatus of claim 15 wherein the biasvoltage waveform periodically returns to ground potential.
 19. Theplasma doping apparatus of claim 15 wherein the bias voltage waveformperiodically returns to a positive potential that at least partiallyneutralize charge accumulating on the substrate.
 20. The plasma dopingapparatus of claim 15 wherein the relative timing of the RF waveform andthe bias voltage waveform is chosen so that the RF waveform has both thefirst and second power levels while the bias voltage waveform is at anegative potential that attract ions in the plasma to the substrate forplasma doping.
 21. The plasma doping apparatus of claim 15 wherein thefirst and second power levels are chosen to increase retained dose inthe substrate.
 22. The plasma doping apparatus of claim 15 wherein therelative timing of the RF waveform and the bias voltage waveform ischosen to increase retained dose in the substrate.
 23. The plasma dopingapparatus of claim 15 wherein the first and second power levels arechosen to enhance knock-on type implant mechanisms so as to produce amore conformal doping profile.
 24. The plasma doping apparatus of claim15 wherein the relative timing of the RF waveform and the bias voltagewaveform is chosen to enhance knock-on type implant mechanisms.
 25. Theplasma doping apparatus of claim 15 wherein the at least one RF antennacomprises a vertical and a horizontal antenna.
 26. The plasma dopingapparatus of claim 25 wherein the RF source applies the RF waveformhaving the first power level to the vertical antenna and applies the RFwaveform having the second power level to the horizontal antenna.
 27. Amethod of plasma doping comprising: a. generating a pulsed waveformhaving at least a first period with a first power level and a secondperiod with a second power level; b. generating a plasma from the pulsedwaveform; c. generating a bias voltage waveform having a first voltageduring a first period and second voltage with a negative potentialduring a second period; and d. applying the bias voltage waveform to asubstrate exposed to the plasma so that ions in the plasma are attractedto the substrate for plasma doping during the second period where thebias voltage waveform has a negative potential, at least one of thefirst and second power levels of the RF waveform being chosen to atleast partially neutralize charge accumulating on the substrate.
 28. Themethod of claim 27 wherein the generating the bias voltage waveformhaving the first voltage during the first period comprises generating apositive voltage that at least partially neutralize charge accumulatingon the substrate.
 29. The method of claim 27 further comprisingadjusting a relative timing of the pulsed waveform and the bias voltagewaveform so that the RF waveform has both the first and the second powerlevel during the second period where the bias voltage waveform has anegative potential.
 30. The method of claim 27 further comprisingadjusting a relative timing of the pulsed waveform and the bias voltagewaveform so as to increase a retained dose in the substrate.
 31. Themethod of claim 27 further comprising selecting at least one of thefirst and the second power levels so as to increase a retained dose inthe substrate.
 32. The method of claim 27 further comprising adjusting arelative timing of the pulsed waveform and the bias voltage waveform soas to enhance knock-on type implant mechanisms to produce a moreconformal doping profile.
 33. The method of claim 27 further comprisingselecting at least one of the first and the second power levels so as toenhance knock-on type implant mechanisms to produce a more conformaldoping profile.
 34. The method of claim 27 wherein the pulsed waveformincludes a third power level having a third period.
 35. The method ofclaim 34 wherein the third power level is zero.
 36. A method of plasmadoping comprising: a. generating a pulsed waveform having a first periodand a second period; b. generating a plasma from the pulsed waveform; c.generating a bias voltage waveform having a first voltage during a firstperiod and second voltage with a negative potential during a secondperiod; and d. applying the bias voltage waveform to a substrate exposedto the plasma so that ions in the plasma are attracted to the substratefor plasma doping during the second period where the bias voltagewaveform has a negative potential, the RF waveform being chosen to atleast partially neutralize charge accumulating on the substrate.
 37. Themethod of claim 36 wherein the pulse waveform has at least two differentpower levels.
 38. The method of claim 36 wherein the pulse waveform hasat least two different frequencies.